1. FIELD OF THE INVENTION
This invention relates to a method of recovering energy from a geothermal reservoir wherein direct heat exchange an immiscible fluid, such as a hydrocarbon, is heated by direct contact with geothermal brine in a second step the heated hydrocarbon is employed in direct heat exchange to heat a second working fluid, such as boiler feed water, to form steam.
2. DESCRIPTION OF THE PRIOR ART
Atmospheric pollution by-products of combustion and thermal pollution by waste heat rejected to rivers and streams complicate the design and operation of fossil fueled power plants. Even nuclear plants, with their inherently higher levels of heat rejection and consequent thermal pollution as well as the ever-present radiation hazard, do not hold promise of providing a source of power free of environmental degradation. Recently, however, geothermal energy has come into prominence as a power source for generating electrical energy with minimal ecological impact. While this energy source cannot be looked to for a solution of the present energy crisis, its utilization would reduce to some extent the environmental degradation resulting from conventional plants.
Although a highly desirable power source, only limited use of geothermal energy is being made at the present time. In the United States, only the Geysers Field north of San Francisco, Calif. is presently being used for commercial power generation. Geological structures making accessible the internal heat energy of the earth are not nearly so rare as would be indicated by the present day usage pattern. Rather, it is the stringent requirements placed on the types of reservoir which may be utilized by application at present technology which has precluded widespread development of the many known areas of geothermal activity. Commercial development has been limited to reservoir in which the heat energy is available in the form of dry steam. In many of the known reservoirs, the heat energy is present in the form of a hot brine solution under sufficient formation pressure to maintain the liquid phase. Depending on reservoir configurations, there may or may not be a steam cap associated with the brine. To date, it has not been feasible to utilize these reservoirs. Direct flashing of hot produced brine has not proven practical because of the severe limitations it places on the energy available in the steam so produced. Indirect heat exchange of the brine against boiler feed water to raise steam would be plagued with exchanger fouling on the brine side at the tubes. Producing steam from the vapor phase section of the reservoir would deplete, at least locally, the water in the brine liquid and precipitate dissolved salts. The precipitated solids would quickly plug the formation, shutting off flow to the producing well.
There is a definite need in the art, therefore, for a process for recovering energy from a geothermal brine in which the brine recovered from the formation after recovery of energy therefrom by a method in which heat exchange surfaces are not fouled by precipitated solids can be returned to the formation so that the reservoir fluids are not depleted.